The present invention relates to the aim of a method for fixing caesium and/or rubidium in a mineral phase of durable containment. More precisely it relates to the fixation of radioactive caesium and rubidium resulting from the reprocessing of irradiated fuels.
The caesium resulting from the reprocessing of irradiated fuels is a long half-life fission product with extremely high volatility and diffusibility. It is thus necessary to fix it in extremely stable matrices.
In the extraction solutions issuing from reprocessing installations of spent fuels, caesium is present under the form of the following isotopes: 135Cs, 137Cs and 133Cs.
Thus, in a solution obtained from a fuel UOx irradiated at 33000 MWj/t, with decay time of 3 years, these three isotopes are found in the quantities shown in the following table 1.
Because of the long-term storage, it is thus necessary to condition the caesium in a matrix which is physically and chemically stable. In fact, this element, not inserted in the network of a defined structure not incorporated in the network of an unstable structure, tends to diffuse outside the matrix under the influence of outside agents, water in particular. Furthermore, during the production of a conditioning material, carried out at high temperature, caesium, which is very volatile, is difficult to incorporate into the matrix.
Effective storage of caesium requires its incorporation into a solid matrix which resists transport, irradiation, which is thermally stable and is inert in geological storage conditions or in long-term storage.
The present policy for conditioning non-separated nuclear waste is vitrification in borosilicate glass. In this case, the caesium is processed together with the other waste whereas it would be of great interest to have specific matrices, specially adapted to the containment of caesium separated from the other waste and/or rubidium.
More recently, it was envisaged conditioning radioactive wastes by coating them in an apatitic matrix possibly containing the actinides and lanthanides to be conditioned, as described in WO-A-95/02886 [1]. However, these matrices were not designed for conditioning caesium and/or rubidium specifically and separately whereas the policy developed nowadays is to manage long-lived radionuclides separately such as Cs. These radionuclides can be separated during reprocessing, under the form of aqueous solutions of caesium and/or rubidium nitrate or carbonate.
The present invention has the specific aim of providing a containment matrix intended for the conditioning of caesium and/or rubidium, with confirmed long-term durability and stability, and which guarantees not only the containment of the waste but also the protection of the environment.
According to the invention, the material for containing the radioactive caesium and/or rubidium comprises a phosphosilicated apatitic matrix including in its chemical structure the radioactive caesium and/or rubidium to be contained, this apatite corresponding to the following formula:
MtCaxLny(PO4)6xe2x88x92u(SiO4)uX
in which:
M represents Cs and/or Rb,
Ln represents at least one trivalent cation,
X represents at least one anion chosen from among 2Fxe2x88x92, S2xe2x88x92, 2Clxe2x88x92, 2Brxe2x88x92, 2Ixe2x88x92, 2OHxe2x88x92 and O2xe2x88x92, and
t, x, y, and u are such that:
0 less than txc2x72.5
2xc2x7xxc2x78
1xc2x7yxc2x77
0xc2x7uxc2x76
x+y+t=10,
and the total number of positive charges provided by the cations M, Ca and Ln are equal to (20+u).
The utilisation in this containment material of a phosphosilicated apatite matrix is very interesting. In fact, 135Cs is a xcex2xe2x88x92 emitter not producing any damage to this matrix, which thus remains stable to these emissions. Furthermore, since the caesium is incorporated in the same network as the apatite, it is thus fixed and cannot diffuse through this matrix. Finally, since apatite is an extremely stable material thermally (up to 1200xc2x0 C.), the 137Cs thermal effect will be of no consequence. As a result,isotopic separation between 135Cs and 137Cs will not be necessary within the framework of caesium conditioning.
Moreover, apatites have very low solubility in water, which diminishes when the temperature rises. This is a positive point for caesium conditioning, since the 137Cs present in the waste, has high thermal power, which implies a rise in temperature of the matrix containing it, but in the case of apatite, will reduce the solubility of the latter in water.
According to the invention, in order to evacuate better the heat involved by the presence of 137Cs in the apatite, its thermal conductivity can be raised by a slight substitution of iron, that is by using, for example, for Ln in the formula given above, a lanthanide and iron.
Phosphosilicated apatites corresponding to the formula given above are especially intended for caesium conditioning, but they are also appropriate for conditioning rubidium. This is particularly interesting in the case where it is not possible to separate the rubidium and caesium present together in an aqueous solution.
According to the invention, the quantity t of caesium and/or rubidium included in the apatite matrix can be varied from 0 atoms per mesh to 2.5 atoms per mesh. A quantity lower than 0.1 atoms per mesh is not of much interest since this formulation only corresponds to 1% by mass of caesium. Generally it is preferable for the quantity of Cs and/or Rb to be less than 1.5 atoms per mesh (txe2x89xa61.5) since an apatite containing a greater quantity, higher than 1.5 atom per mesh is difficult to synthesise because of the high size of the caesium ion.
In this case, there is the risk that the caesium does not enter the apatite network in substitution but in insertion. As a result, because of its high mobility, the caesium would be less linked to the network and could diffuse through the matrix.
Preferably, according to the invention, one uses an apatite in which u is equal to 1, that is a composition containing 5 phosphate groupings and 1 silicate grouping, since studies on the phosphate-silicate solid solution, that is for u ranging between 0 and 6 have shown that the caesium is incorporated best within the apatitic structure for the value u=1. For compositions apart from this value, there is a risk of seeing the caesium crystallise in the secondary phases and being less well included in the chemical structure.
In the phosphosilicated apatite corresponding to the formula given above, the total number of negative charges is brought by the anions PO43xe2x88x92, SiO44xe2x88x92 and X2xe2x88x92. These charges are balanced by the positive charges of Ca2+, M+ and the trivalent cation Ln.
As an example X2xe2x88x92, can represent (FO0,5)2xe2x88x92. For the trivalent cation Ln, various trivalent cations can be used, in particular those belonging to the lanthanide group as well as iron and aluminium. As an example, Ln can be constituted of La alone, La in combination with Fe or furthermore Nd alone.
The present invention also has the aim of a process for containing the caesium and/or rubidium in a phosphosilicated apatite matrix corresponding to the following formula:
MtCaxLny(PO4)6xe2x88x92u(SiO4)uX
in which:
M represents the Cs and/or Rb to be contained,
Ln represents at least one trivalent cation, chosen for example among the lanthanides, iron and aluminium,
X represents at least one anion chosen from amongst 2Fxe2x88x92, S2xe2x88x92, 2Clxe2x88x92, 2Brxe2x88x92, 2Ixe2x88x92, 2OHxe2x88x92 and O2xe2x88x92, and
t, x, y, and u are such that:
0 less than txe2x89xa62.5 and preferably 0 less than txe2x89xa61.5
2xe2x89xa6xxe2x89xa68
1xe2x89xa6yxe2x89xa67
0xe2x89xa6uxe2x89xa66 and preferably u=1
x+y+t=10
and the total number of positive charges provided by the cations M, Ca and Ln are equal to (20+u), which comprises the following stages:
a) preparation of an intermediary compound of theoretical formula:
xe2x96xa1tCaxLny(PO4)6xe2x88x92u(SiO4)uX1xe2x88x92txe2x96xa1t
with t, x, y and u having the meanings given above and xe2x96xa1 representing a lacuna, by mixture of compounds in stoichiometric quantities containing Ca, Ln, P, O, Si and X and calcination of the mixture at a temperature of 1200 to 1500xc2x0 C.,
b) introduction of Cs and/or Rb in this intermediary compound by mixing a powder of this compound of Cs and/or Rb in the quantity required for filling the lacunae, and by calcinating the mixture at a temperature of 700 to 900xc2x0 C., and
c) densification of the britholite containing the caesium and/or the rubidium obtained in b) by pressure sintering at a temperature of 900 to 1050xc2x0 C., under a pressure of 25 to 35 MPa.
This process thus makes it possible to fix and immobilise caesium under atom form to provide a durable containment material.
The first stage of the process consists of mixing the appropriate reagents, that is compounds containing Ca, Ln, P, O, Si and X and calcinating the mixture at a temperature of 1200 to 1500xc2x0 C. In this way an intermediary compound is obtained.
In the second stage of the process according to the invention, the caesium and/or rubidium is introduced into this intermediary compound starting from a compound of Cs and/or Rb which is mixed with a powder of the intermediary compound in the required quantity to obtain a stoichiometric apatite, and the mixture is calcinated at the appropriate temperature.
The compound of Cs and/or Rb can be obtained by drying and evaporation of a solution containing these elements, obtained after selective separation.
The last stage of the process consists of densifying the ensemble by pressure sintering.
In the first stage, one can use various compounds containing two or several of the elements to be introduced, for preparing the intermediary compound. In particular, one uses oxides, carbonates, phosphates and/or fluorides of Ln, Ca and Si.
For synthesising this intermediary compound, one can for example use the following reagents: Ln2O3, SiO2, CaCO3, Ca2P2O7xcex2 and CaF2.
The calcium pyrophosphate Ca2P2O7xcex2 can be obtained by calcination of hydrogenophosphate of anhydrous or dihydrated calcium (CaHPO4 or CaHPO4, 2H2O) at about 1000xc2x0 C., for 1 to 2 hours.
These reagents are then weighed in stoichiometric quantities in function of the chemical composition targeted, then they are mixed in a liquid medium in a liquid which can be acetone, water or alcohol, for example until quasi-total evaporation of the liquid. One can dry at a temperature of 100 to 150xc2x0 C., chosen in function of the liquid used, for example for 2 hours in an oven to dry the powder completely. The mixture is then calcinated at a temperature of 1200 to 1500xc2x0 C. during a sufficient length of time, for example 6 hours. Preferably, the temperature is increased slowly, for example by 10xc2x0 C./min, so as to evaporate all the residues completely. The time or the temperature of calcination can be raised in order to obtain a perfectly crystallised material.
This first stage of the process is easy to operate since it only uses non-radioactive reagents, and therefore does not need any particular preparations concerning radiological protection. The material used, as well as the operating personnel do not have to use any protection. This first stage thus corresponds to a classic industrial synthesis.
In order to implement the second stage of the process according to the invention, first of all the powder of the intermediary compound obtained previously is submitted to grinding so that it has a particle size of less than 10 xcexcm, then the compound of caesium and/or rubidium is added to it, for example a carbonate or nitrate of caesium or possibly rubidium, in stoichiometric quantity, and one mixes intimately in a liquid medium following the same protocol as for the first stage until a perfectly dry powder is obtained. Next one calcinates the powder which is constituted of a mixture of the intermediary compound and reagent with caesium and/or rubidium, at a temperature of 700 to 900xc2x0 C., for example 800 to 900xc2x0 C., depending on the reagent used, for example for 30 minutes to eliminate the CO2 of the carbonate of caesium and/or rubidium or the NO2 of the nitrate of caesium and/or rubidium and to incorporate them in the intermediary compound. Thus one obtains a powder of britholite of caesium and/or rubidium.
Next comes the third stage of the process according to the invention, which consists of densifying the powder obtained. With this aim, first of all the powder is ground, for example for between 3 and 5 hours, in a liquid such as alcohol, water or acetone, by attrition, at a speed for example of the order of 450 revs/min. One can operate in a nylon jar mill with a nylon paddle and balls of 1.5 mm diameter in ceramic such as zircon or zirconia. Thus the size of the grains is reduced to make the powder more reactive. After this grinding, one has a powder of britholite with a specific surface of the order of 5 to 10 m2/gm, for example 6 m2/gm, which corresponds to a particle size lower than 10 xcexcm, and preferably lower than 5 xcexcm.
This grinding operation is indispensable because a non-ground powder has a much lower specific surface, that is a much higher particle size, which would imply pressure sintering at much higher temperatures, which are higher than the volatilisation temperature of caesium. In this case, the latter would not be caught in the apatitic structure and would contaminate the environment.
Ulterior densification of the powder of ground britholite is carried out by pressure sintering in such a way as to give it good mechanical properties. The pressure sintering can be carried out by mono-axial compression. In this case, one can use a mould, preferably in graphite, or a matrix in graphite with a ceramic casing such as alumina or zirconia, the pistons and counter-pistons also preferably being in graphite, but able to be in ceramic such as alumina or zirconia.
In order to aid de-moulding after pressure sintering, the internal walls of the mould as well as the ends of the pistons and counter-pistons in contact with the powder, are coated with lubricants such as nitride of boron or alumina, if necessary.
During pressure sintering, a mono-axial pressure of 25 to 35 MPa is applied to the powder. The pressure sintering cycle consists of raising the temperature to the pressure sintering temperature, for example 900xc2x0 C., followed by a stage at this temperature for about 1 hour. Once the shrinkage is completed, the pressure is withdrawn and the temperature is lowered by 5xc2x0 C./min. At the end of the pressure sintering, the pastille is de-moulded without any difficulty.
Thus one obtains dense britholite containing the caesium to be contained.
This pressure sintering stage makes it possible to obtain a solid without open porosity, with a densification level close to 100%, having contained all the caesium introduced into the apatitic structure phase, chemically stable and which can be placed in a metallic drum for transport and definitive surface storage or in geological storage.
Other properties and advantages of the invention will become clearer by reading the following examples, evidently given as illustrative but non-limiting examples.
The following examples illustrate the preparation of phosphosilicated apatites, starting from the following reagents: Ln2O3, SiO2, CaCO3, Ca2P2O7xcex2 and CaF2 The caesium and rubidium are introduced by way of Cs2CO3, CsNO3 and RbNO3.